The mammalian tridecapetide neurotensin (NT) is a "candidate" hormone of the gastrointestinal tract. It is released into the circulation in response to perfusion of the intestinal lumen with a micellar solution of lipid (4); and a veno-arterial gradient can be demonstrated across the intestine during lipid stimulation. NT itself comprised only 40 percent of the neurotensin-like immunoreactivity detected with an N-terminal directed antiserum in extracts of plasma collected after a physiological stimulus (8). The remainder of the NTLI was composed of readily formed metabolites of NT. Other immunoreactive substances (besides NT and metabolites), measured with a C-terminal directed antiserum, comprised 2/3 of the NTLI measured in extracts of plasma with this antiserum. The physiological effects attributed to NT in the gastrointestinal tract may be due to: 1) action of its metabolites, whose concentrations in extracts of plasma also increase after a test meal; 2) other substances that happen to cross-react in the RIA (particularly with the C-terminal directed antiserum); 3) local (paracrine) activity of NT in the sites where it is released, with its presence in the circulation reflecting its clearance from the site of its action. The biochemical-physiological studies proposed here are designed to elucidate the nature of the circulating NT-like substances and define the metabolic fate of endogenous NT. First, substances will be purified that are found in plasma and that share structural characteristics with the active C-terminal region of the NT molecule and are elevated after lipid perfusion. While these substances may be members of the "neurotensin family", they are structurally distinct from NT, originate in different source tissues, and have never been purified. Second, the release and metabolism of NT by ileal tissue in vitro will be studied using a modified Using chamber to hold the tissue, and established chromatographic procedures to identify the metabolities, to determine whether metabolites may be formed locally, in the absence of the circulation. Finally, a homogeneous cell line of NT-producing cells (derived a rat medullary thyroid carcinoma) will be employed to study the degradation of NH by these cells, to analyze the inhibitory effect of glucagon upon this degradation, and to compare the degradation of exogeneous NT with that of endogenous peptide (synthesis from labelled amino acid precursors) released in response to stimulation.